For joining innovative high-tech composite materials, researchers and engineers have rediscovered the art of sewing.
Composites are increasingly being used by the aerospace industry as a substitute for metals. That is because composites are much lighter – and weight matters. Every kilo that is lost means lower fuel costs or higher payload capacity. About a quarter of the structure of the Airbus A380 will consist of lightweight carbon-fibre- reinforced plastic (CFRP). CFRP materials are being used for the central wing box, the wing flaps, the entire aft fuselage section including vertical and horizontal stabilisers and the rear pressure bulkhead. The latter is the “wall” that closes off the rear end of the cabin, which has to withstand the entire force of internal cabin pressurisation during flight.
While it is important to keep the weight within specification, the manufacturing process must also be cost-effective. For relatively flat components with a large surface area, like the outer skin of the tail unit, resin- impregnated carbon-fibre tapes (prepregs) are employed. Before being hardened by pressure and heat in an autoclave, a pressurised oven, the prepregs are processed to their ultimate shape by computer controlled tape-laying machines – a relatively low-cost process.
But more tightly curved or more complex structures require other manufacturing concepts. New manufacturing methods were therefore developed in a concerted effort by EADS Innovation Works, Airbus and suppliers. Components made of CFRP fabric are processed to the desired shape in the dry state, before being impregnated with resin.
It was at this point that the researchers rediscovered the homely art of needlework. But even this part of the manufacturing process had to be automated, to save time and money. The scientists therefore employ ultra-modern industrial robots to stitch the seams. An added advantage of this technique is that by skilfully varying the position of the stitches it is possible to adapt each part of the component to the stresses likely to occur at that point in later use.
The starting point of the manufacturing process for the A380 rear pressure bulkhead are externally supplied, multiaxial carbon-fibre preforms, which are much easier to handle than sticky prepregs. The individual sheets of composite material are joined together using the automated stitching technique, which enables reliable, reproducible results.
Of course, the “sewing machine” bears little resemblance to the familiar domestic appliance. Several lengths of carbon-fibre fabric are laid side by side on a table measuring over eight metres in both length and width.
A metal crossbeam travels back and forth across the table, carrying the actual sewing head of the machine, with a needle that pulls the thread through the material at a rate of up to 100 stitches per minute. The engineers use the so-called blind stitch and a curved needle capable of working the seam from one side. This arrangement allows almost any length of material to be joined. The complete assembly of joined sheets for the rear pressure bulkhead emerges from this robot sewing machine in the form of a large “carpet”. In the next step, the carpet is rolled up and then rolled out again over a shaped mould that looks rather like a giant upside-down pudding bowl. To obtain the necessary strength from the carbon composite material, six of these carpets are laid in alternate directions on top of one another. The preformed stack of sewn-together, fabric sheets is then placed in an autoclave, where the incorporated film of solid resin melts under the heat and is pressed between the fibres by vacuum, to harden there. Thanks to the high level of automation, the process is extremely cost-effective and very reliable.
Composites are increasingly being used by the aerospace industry as a substitute for metals. That is because composites are much lighter – and weight matters. Every kilo that is lost means lower fuel costs or higher payload capacity. About a quarter of the structure of the Airbus A380 will consist of lightweight carbon-fibre- reinforced plastic (CFRP).
